The present invention relates to a mold and method which are used to cast a metal article as a single crystal.
Molds for casting single crystal metal articles have previously utilized helical crystal selectors of the so-called "pigtail" type. The casting of single crystal articles utilizing a helical selector is relatively expensive. This expense may be contributed to by a substantial scrap rate.
The molds for forming single crystal articles are commonly made by an investment or lost wax process. In this process of making a mold, a wax pattern is first formed. The wax pattern is covered with a layer of ceramic mold material. The wax pattern is removed from the layer of ceramic mold material to leave a space having a configuration corresponding to the configuration of the wax pattern. The forming of the wax pattern with a helical selector requires complicated tooling or an injection mold in which the wax pattern of the helical selector is formed. Single crystal molds having a helical selector are disclosed in U.S. Pat. Nos. 4,475,582 and 4,548,255. A helical selector having a central support element is disclosed in U.S. Pat. No. 4,180,119.
It is frequently desired to cast a doubly oriented single crystal metal article. Thus, the operating characteristics of a single crystal metal article may be enhanced when the article has selected primary and secondary orientations. The primary orientation of a single crystal article is along a primary or Z axis which corresponds to the [001] direction of crystal growth. The secondary orientation of the single crystal article may be along either an X or Y axis. The X axis corresponds to the [100] direction of crystal growth and the Y axis corresponds to the [010] direction of crystal growth.
Although known crystal selectors may tend to promote the growth of crystals having a secondary orientation which is a function of the configuration of the selector passage, the known crystal selectors have not been successful in obtaining doubly oriented single crystal articles, particularly on a production basis. It has previously been suggested that single crystal seeds having a known crystallographic orientation be used to cause molten metal to solidify as a cast article having a desired crystallographic orientation. When this is to be done, the seed crystal is accurately positioned with its primary and secondary orientations corresponding to the desired primary and secondary orientations of the article. Solidification of molten metal to form the article is initiated at the upper side surface of the seed crystal. This may result in the growth of a single crystal having an orientation corresponding to the crystallographic orientation of the seed crystal and the desired crystallographic orientation of the article.
Unfortunately, it is not uncommon for more than one crystal to be nucleated at or adjacent to the seed crystal. Spurious or unwanted nucleation of secondary crystals may be caused by contaminants or other imperfections which occur during casting, particularly during casting on a production basis. The extra or secondary crystals will usually have a random crystallographic orientation which is almost always different than the desired crystallographic orientation of the article to be cast. Therefore, it is important that the secondary or spurious crystals do not grow into the article mold cavity.
If the secondary or spurious crystals grow into the article mold cavity, they can result in the formation of an article which is not a single crystal article. Thus, the secondary crystals can result in the formation of an article which is composed of two or more crystals having different crystallographic orientations. The secondary crystals can also result in the formation of an article having a crystallographic orientation which is different than the desired crystallographic orientation.
In order to enable articles to be cast with the desired primary and secondary crystallographic orientations, it has been suggested that a crystal selector be used in combination with a seed crystal. Thus, U.S. Pat. No. 4,714,101 discloses the use of a straight vertical crystal selector to block the growth of crystals nucleated along the sides of a seed from entering an article mold cavity. In addition, U.S. Pat. No. 4,475,582 discloses the use of a helical or "pigtail" type crystal selector in combination with a seed crystal.